In many types of fluid systems, it is necessary to modulate or otherwise control a flow of fluid through a fluid circuit. For example, heating ventilation and air conditioning (HVAC) systems for building typically include multiple fluid circuits providing heated or chilled water to heat exchangers within the building. The flow of fluid in the circuits is generally modulated by one or more valves in each fluid circuit, which are operatively connected to and controlled by a control device, such as a thermostat, to provide a flow of fluid to the heat exchangers which will result in a desired temperature being maintained in inside the building.
As part of the process of selecting a control valve for use in a particular fluid system, the relationship between the effective flow area of the control valve, relative to the degree of opening, is known as the “valve characteristic.” For valves used in HVAC systems, it is typically desirable to have a so-called “equal percentage” valve characteristic, over a desired range of fluid flows. In a valve exhibiting equal percentage characteristics, a change in the degree of opening of the valve (as a percentage) with respect to a previous given degree of opening, will result in an equal percentage change in fluid flow over the fluid flow at the previous given degree of opening. For example, if opening the valve by an additional 10% causes a corresponding 10% increase in fluid flow, the valve exhibits equal percentage characteristics. A valve with equal percentage flow characteristics increases fluid flow at a very low rate when the valve first begins to open, and then, as the degree of opening becomes larger, the rate of increase in fluid flow for an incremental increase in opening becomes larger.
It is understood by those having skill in the art, however, that, while having a valve providing a true equal percentage valve characteristic is a desirable theoretical goal, most actual control valves do not inherently exhibit such characteristics over their entire operating range. For example, although it is often desirable to use ball valves in such HVAC systems, for various reasons including reliability, small size and relatively low cost, the valve characteristics of ball valves are not generally inherently well suited for use in HVAC applications.
A typical ball valve includes a valve member, generally in the form of a spherical ball, which is rotatably mounted between two seals in a valve housing. The valve housing defines a flowpath extending upstream and downstream from the valve member, and the valve member includes a bore extending therethrough that can be selectively aligned with the flowpath for regulating fluid flow through the flowpath.
Both the bore through the valve member, and the flowpath in the housing typically have circular cross sections. As the valve member is rotated through an angle of 90 degrees, the bore moves from a fully open position, in which the bore is fully aligned with the flowpath, to a fully closed position, in which the bore extends perpendicularly to the flowpath, with both ends of the bore located between the two seals, so that no fluid can flow through the ball. As the bore opens, a leading edge of the bore forms a controlling edge of the outer surface of the ball, which moves transversely across the flowpath as the valve member is moved from the fully closed toward the fully open positions.
The transverse position of the controlling edge in the flowpath defines an effective axially opening area of the flowpath, which appears in cross section to be generally “football shaped” having two oppositely pointing ends joined by a pair of oppositely outwardly curved edges which form a shape that is wider in the middle than at the pointed ends. As the controlling edge moves transversely to the flowpath, the football shaped area opens and closes rather rapidly, as the ball is rotated, particularly when the valve first begins to open.
Although this rapid change in area is a desirable characteristic in ball valves used in on-off type applications, where the ball valve is simply moved from the fully open position to the fully closed position, such rapid changes in cross sectional area are not typically desirable in applications, such as HVAC systems, where the ball valve is to be used for modulating fluid flow at partially open positions of the valve. This rapid change in area also does not provide a desired equal percentage flow characteristic over any portion of the operating range of the ball valve, between the fully open and fully closed positions, making a ball valve an inherently poor choice for use as a modulating valve in an HVAC system.
Through the years, however, manufacturers of ball valves have learned that by adding a flow characterizing device adjacent to the valve member of the ball valve, or within the bore of the valve member, the inherent flow characteristics of the ball valve can be modified to provide significantly improved performance in ball valves used as modulating control valves in HVAC systems. Through the use of such flow characterizing devices, an equal percentage valve characteristic can be sometimes be provided over at least a portion of the operating range of the ball valve.
Prior flow characterizing devices have often included a wall, extending across the flowpath, having a surface that is configured to closely conform to the outer surface of the valve member. The wall includes a specially shaped opening, that restricts the cross sectional area for fluid flow, to less than that which would otherwise be inherently presented by the degree of alignment of the bore in the valve member with the flowpath, in such a manner that the rapidly opening and closing characteristics of the ball valve are modified to provide a flow characteristic that better resembles a theoretical equal percentage flow characteristic, or some other flow characteristic which may be more desirable for a particular application than the inherent rapid opening and closing characteristic of a ball valve that does not include a flow characterizing device.
Through the years, prior flow characterizing devices have utilized single or multiple openings in a wide array of shapes and sizes extending through the wall. Where it is desired to provide an equal percentage characteristic over at least a portion of the valve operational range, a single elongated opening has often been utilized, with the elongated opening extending transversely across the flowpath from an apex located near the fully closed position of the controlling edge, (i.e. the point at which the bore in the valve member first begins to open or is fully closed) to an opposite, considerably wider, end of the elongated opening that is located near the fully open position of the controlling edge.
Between the apex and the opposite end, the sidewalls of the elongated opening have taken many shapes, through the years, in prior valve characterizing devices. In an early approach, as exemplified by U.S. Pat. No. 3,563,511, to Bentley-Leek, a flow characterizing insert, having a generally triangular shaped opening, similar to the opening shown in FIG. 1A, was utilized. In a later approach, which is still in use today, Worcester Controls offered a line of “Characterized Seat Valves,” in which one of the seals of a ball valve included a wall defining an elongated opening in one of a variety of shapes, and combinations of shapes, including slots, holes and/or angled sides, as shown in FIGS. 1B–F, extending across the flowpath. More recently, others have proposed using elongated holes having side walls forming simple parabolic or other curved shapes, as shown in FIG. 1E, and exemplified by U.S. Pat. No. 5,937,890, to Marandi
In even more recent prior approaches, as shown in FIGS. 1F–H, and exemplified by published U.S. patent application number US 2001/0030309 A1, to Carlson, et al; and U.S. Pat. No. 6,109,591, to Tuttle, et al; flow characterizing devices in the form of inserts or bearings included openings having side walls of complex curved shapes extending from a narrow apex, or pointed end, that was disposed adjacent the fully closed position of the controlling edge, when the flow characterizing device was installed adjacent the outer surface of a ball valve. In such arrangements, the opening is typically very narrow adjacent the apex, and then diverges rapidly in a smooth curve to provide a substantially wider open area adjacent an end of the opening opposite the apex.
The continual development, spanning several decades, in the shape and complexity of openings in flow characterizing devices is indicative of the practical difficulties involved both in designing and producing a characterizing device having an opening shape that will provide a desired valve characteristic, which may in some cases include at least portion thereof that approximates an equal percentage characteristic.
Designing such openings is made difficult by several factors, particularly where it is desired to have an equal percentage valve characteristic. In addition to the requirement that a valve characteristic provide an equal percentage characteristic over at least a desired range of controlled flows, it is generally a requirement that a ball valve provide a desired valve coefficient (CV) at a fully open position, and that the valve be capable of completely shutting off flow at the fully closed position of the valve member. In general, it is desirable that the valve be capable of supplying almost full flow at openings of 80% or greater, and provide for precise control of flow at valve openings between 0% and 80%, preferably according to an equal percentage characteristic for valves used to control flow in HVAC systems.
It is so difficult, in fact, to simultaneously meet all of these requirements in a single theoretical equal percentage characteristic curve, that designers typically utilize modified characteristic curves, having different incremental percentages for valve openings in the region from 80% to 100% of fully open, than are used in the range of 10% to 80% of fully open. Because a true equal percentage curve will never decrease totally to zero from any starting point at which the valve is open, designers must also typically modify the theoretical characteristic curve in some manner, for the range of valve openings below a low value, such as 10%, to cause the valve to close fully. Mathematical curves describing such modified equal percentage curves can be quite complex to develop, as exemplified in the US patent application referenced above, to Carlson et al, by a device “having a cross sectional area which approximates ea (h/100−1), where a is between about 2 and 5, and h is the valve shaft position, but modified so that the cross-sectional area is zero, when h is zero.”
Even after such complex mathematical descriptions are developed by the designer, many practical difficulties exist which make it difficult to actually produce a flow characterizing device that will perform in accordance with the theoretical curve. Considerable difficulty is created by the very small, narrow, openings that are required adjacent the apex of the openings in prior characterizing devices. Complex manufacturing processes are typically required, such as cutting with a computer guided Laser or EDM (Electrical Discharge Machining), to accurately hold the tolerances on the dimensions of the complex shaped openings within tight enough limits to achieve the performance predicted by the theoretical characteristic curve developed by the designer. In some prior approaches, as exemplified by in the U.S. patent to Tuttle, et al, referenced above, it is suggested that the a valve seat be constructed of two halves which are lapped to fit closely together and secured together in a precise inter-fitting relationship, with corresponding portions of an aperture being formed in each half of the two halves.
Carlson, et al, teaches that, for very small flows, the opening tends to be very narrow adjacent the apex, creating a risk that dirt particles or other contamination may accumulate and interfere with operation of the valve. Carlson discloses disposing a cover or “tent” over the apex, leaving the flared ends uncovered, so that the cover prevents fluid from flowing directly from one side of the disk to the opposite side. Instead, the fluid has to flow sideways to find the portion of the opening that is not covered. The fluid has to flow a relatively long way before it passes the disk. Thus, according to Carlson, the cover enables the use of a larger opening near the apex while maintaining the desired flow characteristics. Carlson further asserts that the larger the opening adjacent the apex, the easier it will be for particles to pass, and that the cover maintains the desired flow characteristics while minimizing particle accumulation. Carlson does not, however, disclose a desired shape for the cover and the opening adjacent the apex, in order to provide flow control when the port in the ball is positioned to exchange fluid only with the covered portion of the opening, so that all flow passing through the disk must pass through the covered portion of the opening.
In addition to the problems involved in holding the tight tolerances that are required on the dimensions of the elongated opening, prior valves have also required a close conformance between the inner surface of the wall of the flow characterizing device and the outer surface of the valve member. As stated in the U.S. patent application to Carlson, et al, cited above, the “[s]urface of the disk that faces the ball advantageously is concave and substantially corresponds to the spherical surface of the ball or plug inside the valve. The disk is preferably mounted with its concave surface resting on or, more preferably, very close to the ball or plug. Preferably, a space between the disk and ball or plug is left so as to minimize fluid from flowing between the disk and the ball or plug (i.e., by-pass flow) yet so as to avoid interference of the disk with the ball or plug and to allow smooth operation of the valve. Most preferably, the space ranges from about 0.0005 to 0.0015 inches, and more preferably is about 0.001 inches.” Fabricating and mounting flow characterizing devices meeting such tight requirements for conformance is a difficult manufacturing task.
What is needed, therefore, is an improved apparatus and method for providing a flow characterizing device for a fluid control valve, meeting the requirements and overcoming one or more of the problems described above in relation to the prior art. It is also desirable to provide such an improved apparatus and method in a form that results in reducing the torque required for repositioning the valve member in a ball valve. This is especially true for ball valves having an actuator motor connected to the valve member for repositioning the valve member, because lowering the torque requirement will allow a smaller actuator to be utilized. Generally speaking, smaller actuators can be produced at lower cost than larger actuators, and require less input power, thereby reducing both the initial cost and the operating cost of the actuator.